Bone screw retaining system

ABSTRACT

A spinal implant has a bone plate with a bone contacting surface and an opposite upwardly facing surface and at least one opening with a groove formed around an inner circumference of the opening. The groove has an axially spaced upper and lower surface. A bone-anchor or bone screw is capable of being accommodated in the opening between the groove and the bone contacting surface. A split-ring is mounted in the groove. The split-ring is sized to expand upon insertion of the anchor element into the opening and to come into direct contact with a head of the bone anchor to hold the bone anchor in the opening. The split-ring has an upper surface and a lower surface for respectively engaging the upper and lower surface of said groove, wherein the split-ring has a variable width so as to optimize its flexibility.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/985,733, filed on Jan. 6, 2011, which is a continuation ofU.S. patent application Ser. No. 11/060,171, filed on Feb. 17, 2005, nowU.S. Pat. No. 7,887,547, which is a continuation of U.S. patentapplication Ser. No. 10/331,212, filed on Dec. 30, 2002, now abandoned,which is a divisional of U.S. patent application Ser. No. 09/665,530filed on Sep. 19, 2000, now U.S. Pat. No. 6,602,255, the disclosures ofwhich are incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to osteosynthesis devices for the spinalcolumn, the devices comprising a plate and a mechanism for locking abone screw or anchoring member in position. This divisional applicationrelates to tools used to remove a bone screw after it has been locked inthe plate.

U.S. Pat. No. 5,876,402 relates to an osteosynthesis plate comprisingthrough-holes of conical shape capable of housing a bone screw with acompletely spherical head to which is clipped, so as to form aball-joint connection. A split coupling element of conical exteriorshape complementing that of the hole is provided. A circlip reduces theaperture of the through-hole. A similar clip and groove arrangement isshown in U.S. Pat. Nos. 5,879,389 and 6,102,952.

In U.S. Pat. No. 5,876,402, the bone screw is placed in the couplingelement prior to insertion in the plate. Upon insertion, the splitcoupling element opens up the circlip. This circlip closes up again oncethe coupling element has passed through. The coupling element is thusheld captive in the through-hole. Final clamping of the anchoring memberin position is achieved by the frictional wedging of the couplingelement in the bottom of the cone.

In such a system, the number of parts makes the clamping-in position ofthe anchoring members weak. In addition, the clamping does not occur atthe instant when the circlip closes up again after the passage of thecoupling element. This leads to the risk of the assembly becomingunclamped, which is prejudicial to the patient.

FIGS. 5 and 6 of U.S. Pat. Nos. 5,879,389 and 6,102,952 show asplit-ring for installation in a groove after the bone screw or anchorhas been installed in the bone.

BRIEF SUMMARY OF THE INVENTION

One of the objects of the present invention is to provide a spinalimplant which is easier to fit while at the same time being reliable.

With a view to achieving this objective, the present invention envisagesan implant, particularly for the spinal column, comprising a joiningmember such as a plate exhibiting openings or orifices, bone-anchoringmembers such as bone screws capable of being accommodated in theorifices and at least one split ring capable of holding the members inthe orifices. The split ring can come into direct contact with theanchoring member or members to hold the member or members in theorifices.

Thus, the number of parts involved in the locking is reduced and thislocking can be made more reliable. Advantageously, the joining membercomprises a plate and the orifices comprise an opening with a sphericalseat.

Preferably, each anchoring member or bone screw comprises acomplementary spherical part capable of coming into contact with thespherical seat. Thus, the surgeon has, at his disposal, freedom toorient the anchoring member angularly with respect to the joining memberor plate, thus allowing him to optimize the anchorage.

Advantageously, the anchoring members or bone screws comprise drivingmeans such as a drive socket.

In one embodiment the split ring is preferably common to at least twoorifices and includes a driving means, which driving means compriseopenings. In another embodiment, the split ring is specific to eachorifice in the plate.

Advantageously, the split ring has a variable cross-section so as tooptimize its flexibility. Thus, the ring will deform more readily whenintroducing the head into the orifice. The amount of time taken and thenumber of operations required during surgical intervention will bereduced.

The bone plate, screw and ring may be supplied as part of a screwlocking system for bone plates to be used by a surgeon. The bone platehas at least one opening therein, and normally a plurality of openings,for receiving a bone screw or bone anchor. The openings extend along anaxis from a top surface to a bottom bone contacting surface of theplate. Each opening has an upper region with a first diameter with agroove formed therein having a depth defined by a diameter greater thanthe first diameter. The plate has a lower region including a seat forthe bone screw. The bone screw has a head with a maximum diameter whichis smaller than the first diameter, thereby allowing the screw head topass through that region of the opening.

An expandable ring is provided which is pre-mounted in the groove andhaving, when relaxed and unexpanded, an external diameter greater thanthe first diameter, but smaller than the groove diameter. The expandablering has an internal diameter when relaxed and unexpanded, smaller thanboth the first and the head diameters. The expandable ring is capable ofexpanding into the groove so that the internal diameter expands to belarger than or equal to the screw head diameter while, at the same time,the external diameter is less than or equal to the groove diameter.

With this geometry, the split-ring can be pre-mounted in the groove andthe screw can be inserted, shank first into the bone plate from theupper non-bone contacting surface and, upon engagement between the headof the screw and the split-ring, the split-ring expands into the groove,allowing the head to pass therethrough. Once the screw head has passedthrough this split-ring, it contracts under its natural spring tension.When the ring relaxes to its unexpanded state, it prevents the bonescrew from backing out of the plate by the engagement of an undersurfaceof the split-ring and an upwardly facing surface on the bone screw.

The openings in the lower portion of the bone plate have a partspherical seat portion located between the groove and the bottom bonecontacting surface of the plate with an opening in the bottom platesurface to allow the shank of the bone screw to pass through. The screwhead has a corresponding part spherical surface extending from the shankof the screw towards the upwardly facing surface of the screw. Uponinsertion of the screw through the plate, the screw head engages thepart spherical seat on the bone plate. At that point the screw head isbelow the split-ring groove. The bone screw shank can be threaded in anywell known fashion and may include an axial groove to enable the screwto be self-boring and self-tapping. The bone screw may include aninternal bore extending along the longitudinal axis of the screw whichincludes threads for engaging a pull out tool should removal of thescrew be necessary.

In order to enhance the locking system's ability to prevent the screwfrom backing out of the bone plate, both the groove and split-ring havecomplementary inclined surfaces extending towards the upper surface ofthe bone plate upon moving towards the center of the opening in theradial direction. The engagement of the surfaces in combination with aforce exerted by the screw on the bottom surface of the split-ringcauses the internal diameter of the ring to decrease with increasingforce from below. This insures the bone screw cannot back out of theopening.

In order to make the insertion of the bone screw easier, it is providedwith an inclined surface complementary to an inclined surface on theinternal bore of the split-ring, which inclined surfaces increase indiameter upon moving in a direction from the bottom surface of the platetowards the upper surface of the plate and radially outwardly of theopening central axis. Thus, when the screw head inclined surface engagesthe complementary inclined surface on the internal diameter of thesplit-ring, forces are generated which expand the split-ring into thegroove. In order to increase the flexibility of the split-ring, at leastone cutout and preferably three or more cutouts are spaced around theexternal diameter of the ring, resulting in a variable cross-section.This allows the ring to have more flexibility in expanding than if theexternal diameter of the ring were constant. In order to better preventthe egress of the bone screw from the plate, the surface of thesplit-ring facing towards the bottom of the plate is flat and extendsgenerally perpendicularly to the central axis through each opening. Thebone screw has a complimentary upwardly facing generally flat orslightly inclined surface.

The location of the groove in the plate is such that when the head ofthe screw fully engages the spherical seat in the plate, the upwardlyfacing surface is located below the bottom surface of the split-ring. Inorder to allow the bone screw to rotate from side to side once seated,an angular cutout of 0° to 20° can be provided at the bottom surface ofthe plate, thereby making the opening on the bottom surface oblong in atleast one direction. This allows the longitudinal axis of the screw headand shank to be rotated between 0° and 20° with respect to the centralaxis of the opening.

The material for the split-ring must be flexible and be compatible withthe body and it has been found that the titanium alloy disclosed in U.S.Pat. Nos. 4,857,269 and 4,952,236, which have modulus of elasticity notexceeding 100 GPa, is acceptable. Polymeric materials such as ultra-highmolecular weight polyethylene are also acceptable.

The joining member or plate may be curved to match the anatomicalcurvatures. Thus, the implant curved to best suit the anatomy andnatural curvature of the spinal column in the case of a spinalapplication. Of course, the plate may be used in fracture fixation, as atibial baseplate, as a hip side plate or any application where boneplates and screws are used. For these uses, a larger screw than thatdescribed herein is necessary. The screw locking system can be scaled upfrom that described herein so that any size screw can be utilized in asmaller locking system.

Also envisaged is a method for implanting the implant involvingaccessing the spinal column via an anterior route, fitting the implant,preparing the anchorage, fitting the anchorage members, locking theimplant and the head of the anchoring members with respect to thejoining member, and closing up the access route.

These and other objects and advantages of the present invention willbecome apparent from the following description of the accompanyingdrawings. It is to be understood that the drawings are to be used forthe purposes of illustration only and not as a definition of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become more apparenton reading the description which follows of the preferred embodimentswhich are given by way of non-limiting examples.

FIG. 1 is a perspective view of a first embodiment of the invention;

FIG. 2 is an exploded perspective view of the first embodiment;

FIG. 3 is a cross-sectional view along lines III-III of the firstembodiment of FIG. 1;

FIG. 4 a is a plan view from above of the first embodiment;

FIG. 4 b is an elevation view of the first embodiment shown in FIG. 4 a;

FIG. 4 c is a front view of the first embodiment shown in FIG. 4 a;

FIG. 5 is a perspective view of a second embodiment of the invention;

FIG. 6 is an exploded perspective view of the second embodiment;

FIG. 7 is a partial view in section on the plane VII-VII of the secondembodiment;

FIG. 7 a is a cross-sectional view of a bone screw or anchor of thepresent invention;

FIG. 7 b is a cross-sectional view of a single orifice used in thesecond embodiment of the invention without the screw and split-ringalong line VII-VII of FIG. 6;

FIG. 7 c is a plan view of the split-ring of the second embodiment ofthe present invention;

FIG. 7 d is a cross-sectional view of the split-ring of FIG. 7 c alonglines A-A;

FIG. 8 a is a plan view of the second embodiment of the invention;

FIG. 8 b is an elevation view of the second embodiment shown in FIG. 8a;

FIG. 8 c is a front view of the second embodiment shown in FIG. 8 a;

FIG. 9 is a perspective view of a third embodiment of the invention;

FIG. 10 is a plan view of a screw driver for driving the bone screws ofFIG. 7 a from the orifice of FIG. 7 b;

FIG. 10 a is an end view of the screw driver shown in FIG. 10;

FIG. 11 is a plan view of an extractor tube for extracting the anchor orbone screw from the plate after implantation;

FIG. 11 a is an end view of the extractor shown in FIG. 11;

FIG. 11 b is an enlarged detail of the drive and of the extractor shownin FIG. 11; and

FIG. 12 is a plan view of a threaded extraction shaft designed to engagethe screw and pull it axially out of the hole, should it be impossibleto unscrew the threaded shank.

DETAILED DESCRIPTION

With reference to FIGS. 1 to 4 c, there is shown the implant accordingto the first embodiment comprises a plate 1, bone screws 5 and circlipsor split-rings 4. The plate 1 is a bone plate such as an anteriorcervical plate or any other plate designed to be held on bone by bonescrews. Plate 1 may join two bone parts or stabilize a fracture or maysit on a resected bone surface such as on a tibial plateau.

In the preferred embodiment, plate 1 is formed of a body 11 ending intwo ends 12 which have a width slightly greater than that of a mid-zoneof body 11. Each of the ends 12 comprises a pair of openings or orifices2 which pass through the entire thickness of plate 1. The four openingsare arranged geometrically as at four corners of a rectangle. Each ofthe openings 2 has a first, upper, cylindrical part 23 which continuesin the form of a spherical central part 21 and ends in a second, lower,cylindrical part 22, the diameter of which is smaller than that of thefirst cylindrical part 23. The spherical intermediate part 21 allows theangle of the bone screw 5 that is to be accommodated in the opening 2 tobe chosen.

Plate 1 preferably comprises two blind holes 3 which have a circularopening and a recess 31. The two blind holes 3 are arranged on thelongitudinal mid-segment of the rectangle, near the respective pairs ofcorners. Recess 31 is such that it protrudes into the pair of openings 2to which it is adjacent, thus creating an open slot 32 in each opening 2of the pair. This slot 32 is made in such a way that it is located inthe first cylindrical part 23 of the openings 2.

Plate 1 has a first curvature 13 in its longitudinal plane, as depictedin FIG. 4 c. This curvature 13 allows plate 1 to follow the naturallordosis of the section of spine for which the plate 1 depicted in FIGS.1 to 4 c, is intended. In addition, the plate 1 has a second curvature14 in its transverse plane as depicted in FIG. 4 b. This curvature 14allows plate 1 to match as closely as possible the shape of the body ofthe vertebra to which it is connected.

Each recess 31 is capable of housing a circlip 4. The circlip 4 is inthe form of a circular ring 43 split at 42. The circlip or split-ring 4comprises driving means 41 which, in this embodiment, are lugsprojecting towards the inside of the ring. Each lug may be shaped toreceive the tips of a pair of needle nose pliers (See FIG. 9).

Once in place in the recess 31, with the circlip 4 in the position ofrest, i.e. in the open position, it protrudes into the pair of openings2 adjacent to it through the slot 32 of each opening 2. It thus closesup the opening 2 slightly.

The bone screw 5 is the preferred anchoring member in the embodimentwhich allows the plate 1 to be connected to the bodies of the vertebraewhich are fitted with the present invention. The preferred screw 5 has ahead 57 surmounting a cylindrical part or shank with a thread 51 suitedfor bone, comprising a self-tapping means 55 at its distal end. Thesetapping means allow the screw to better penetrate the bone when beingdriven. The head 57 comprises a drive 52 which, in this instance, isembodied by a hexagonal socket. In addition, the head 57 comprises aslightly conical part 53 which is continued in the form of a part 56forming a rim extending towards the outside of the screw 5 and inclinedslightly with respect to a plane perpendicular to the axis A of thescrew 5.

Finally, the head 57 of the screw 5 ends in a spherical male part 54which complements the female intermediate part 21 of the opening 2 andwhich meets the threaded cylindrical part or shank 51. Thesecomplementing forms allow the bone screw 5 to be set at a chosen anglewith respect to plate 1. The anchoring of plate 1 can thus be optimizedby the surgeon during the operation.

Preferably, the implant of the present invention shown in FIGS. 1-4 issupplied to the surgeon with the two circlips 4 installed in recesses 31of plate 1. If the plate is an anterior cervical plate, it is preferablyimplanted by an anterior access route and by uncovering the vertebralbodies that will be fitted. The surgeon positions the plate 1 thenpierces pilot holes through each pair of openings where he wishes tohave an anchorage. He then engages a bone screw in each pilot hole. Hescrews these in until the part 54 of their head 5 comes into contactwith the part of the ring 44 of the circlip 4 that projects through theorifice 32. At this point, there are two possible options:

1. The surgeon closes up the circlip 4 by bringing the two lugs 41closer together using pliers and then, holding the circlip closed, hescrews the two bone screws 5 in until the complementary spherical parts21 and 54 come into contact, then, releasing the circlip which returnsto the open position over the rim 56;

2. The surgeon continues to screw in the anchoring member 5, thespherical part 54 pushing the ring 44 into the slot 32 through a rampeffect and thus forming its passage, and the ring will open againautomatically once the rim 56 has passed by, and the complementaryspherical parts 21 and 54 will be in contact.

Locking is provided by contact between the complementary spherical parts21 and 54 and by the re-opening of the circlip 4 above the rim 56. Thesecond role of the rim 56 is to limit the possibilities of angularorientation. This prevents the screw from coming out of the vertebralbody or from coming into contact with its counterpart fitted in theother opening 2 forming the pair. In both instances, the plate would bepoorly anchored or even not anchored to the vertebral body at all. Thus,having introduced each screw into the orifice via its distal end, thecirclip prevents the screw from backing out of the orifice.

In the event of an adjustment, the surgeon can easily withdraw the plate1 simply by unscrewing the bone screws 5 after having closed up thecirclips 4 by moving their lugs 41 closer together, thus uncovering theaperture of the orifice 2.

In a second embodiment illustrated by FIGS. 5 to 8 c, cervical plate 1is preferably still formed of a body 111 ending at two ends 112 whichare slightly wider than the body 111. Each of the ends 112 still has apair of openings 102 which pass right through the entire thickness ofthe plate 101. Each opening 102 has a first part 123 which iscylindrical, then a spherical intermediate part 121. Preferably, theorifice or opening 102 has a part 122 in the form of an angular cutoutin the lengthwise direction of the plate 101. Preferably, the cutoutallows the screw to pivot an angle B, preferably from 0° to 20°, in thelengthwise direction about axis 164, preferably the width of the cutout122 is slightly less than its length. A circular recess or groove 131 isformed in the cylindrical part 123 of each opening 102. As in theprevious embodiment, when used as an anterior cervical plate, the plate101 has a curvature 13 in its longitudinal plane and a curvature 14 inits transverse plane. The roles of these curvatures are the same as inthe previous embodiment.

The recess 131 is able to accommodate a circlip or split-ring 104. Asbefore, the circlip 104 is in the form of a circular ring 143, split at142. The preferred circlip or split-ring 104 in this instance has tabs141 and cutouts 149 distributed uniformly around the entirecircumference of the ring 143. Preferably, there are at least 3 of thesetabs. They make it possible to be sure that the circlip will not escapefrom the groove or recess 131, while leaving thinner parts of the ring143 to allow better flexibility when deforming or expanding the circlipas will be discussed hereinbelow. Of course it is possible to make thering thinner or use other means to achieve flexibility in the ring. Forexample, one or two tabs could be used if the cutouts in the ring aresized sufficiently to produce the required flexibility. The circlip 104comprises expansion chamfer or ramp 144 in the form of an inlet chamferlocated on the interior side 145 of the ring 143.

The bone screw 105 differs from that of the previous embodiment in thatthe drive 152 which is in the form of a cross is extended in oneembodiment by a blind bore 158 coaxial with the axis A of the screw 105.This allows the use of a screwdriver with a flat or cruciform bladeextended by a small cylindrical protrusion that complements the blindbore 158. Thus, when tightening or loosening, the screwdriver cannotslip to injure nearby living tissues or irreversibly deform the circlip104, as this would compromise locking.

The preferred head 157 has a part 153 which is generally conical andwhich slightly bows outwardly which is continued radially outward by apart 156 forming an upwardly facing rim surface extending towards theoutside of the screw 105 and which preferably is slightly inclined withrespect to a plane perpendicular to the axis A of the screw 105.

Finally, a part spherical portion 154 that complements the intermediatepart 121 of the opening 102 allows the outer edge of the portion 154 tomeet the threaded cylindrical part or shank 151, which is threaded witha bone-screw thread. The purpose of this complementing nature is toallow the angle of the screw 105 to be chosen with respect to the plate101 in order to optimize anchorage.

As in the previous embodiment, the implant is supplied to the surgeonwith the four split-rings or circlips 104 installed in the four recesses131 in the plate 101. As before, the surgeon, having made his accessroute, then positions the plate 101 and pierces the pilot holes throughthe pairs of openings 102 where he wishes to anchor, completely screwsin the bone screws 105. At the end of tightening, the spherical part 154will come into contact with the chamfer 144 of the circlip 104 and then,through a bearing action, open up the latter to make its passage towardsthe spherical intermediate part 121 of the orifice 102. The circlip 104will close back up again automatically once the rim 156 has passed, andthe complementary spherical parts 121 and 154 will be in contact.

Performing these two operations makes sure that the screw 105 is lockedin the plate 101. As before, the second role of the upwardly facing rim156 is to limit the possibilities of angular adjustment. This preventsthe screw from coming out of the vertebral body or its threaded shank151 from coming into contact with its counterpart fitted in the otherorifice 102 forming the pair. In both instances the plate would bepoorly anchored or not anchored to the vertebral body at all. In theevent of an adjustment, the surgeon can easily withdraw the plate 101simply by unscrewing the bone screws 105 after having opened up circlip104 as will be discussed below.

A preferred bone screw of the second embodiment of the present inventionis shown in greater detail in FIG. 7 a. In the preferred embodiment, theblind bore 158 of screw 105 is threaded for a portion 160 located belowdrive 152 towards the tip 161 of the screw. The function of the threadedportion will be described in greater detail below.

It should be noted that the preferred screw 105 has a nominal threaddiameter of about 4 mm with the outer diameter of the upwardly facingsurface rim 156 being about 5 mm. If desired, the leading end or tip 161of the screw shank 151 may include a groove or other structure forallowing the bone screw to be self-drilling and self-tapping. In thissituation, no pilot hole need be drilled by the surgeon.

Referring to FIG. 7 b, there is shown the preferred opening 102 in thebone plate. The recess or groove 131 which accommodates spring clip 104has an upwardly and inwardly inclined surface 133 which, in thepreferred embodiment, extends at an angle of about 20° with respect tothe bottom surface 135 of groove 131. In the preferred embodiment, thebottom surface 135 of groove 131 extends along a plane perpendicular tothe axis 164 of the opening 102. The upper inclined surface 133 isspaced from surface 135 by surface 137 which, in the preferredembodiment, is about 0.3 mm. The maximum diameter to surface 137 ofgroove 131 is, in the preferred embodiment, about 6.9 mm. Spherical seat121 for screw head 157 extends from adjacent the bottom bone contactingsurface of the plate to surface 135. In the preferred embodiment, thespherical surface has a radius of 2.67 mm. Consequently, the partspherical portion 154 of the screw has a similar radius. As can be seenin FIG. 7 b, the opening 102 may have an angular cutout along a portionof surface 122 adjacent the bottom plate surface to allow the shank 151of the screw to extend in at least one direction at an angle B ofapproximately 0° to 20° and preferably 10° with respect to the axis 164.Thus, when viewed from the bottom, the opening would appear to be oblongin at least one direction. Of course, the angular cutout can be enlargedto permit angulation in a plurality of directions.

Referring to FIG. 7 c, there is shown a preferred split-ring or circlip104 which includes five tabs 141 distributed uniformly around thecircumference of the ring 143. In the preferred embodiment, the ring hasan internal diameter 145 of approximately 4.5 mm and a maximum externaldiameter 147 of preferably 6.2 mm. The difference between the externaldiameter 147 and the groove diameter 137 is preferably about 0.7 mm.This allows the internal diameter to expand to accommodate the screwhead. The preferred cutouts have a depth of approximately 0.4 mm so thatthe external diameter 149 at each cutout is approximately 5.4 mm. Thepreferred split 142 is 0.26 mm in width when the split-ring is in itsrelaxed, i.e. unexpanded condition. The above dimensions are given forillustration only and larger screws, openings and split-rings may beused in other applications.

Referring to FIG. 7 d, there is shown a cross-section of the split-ringshown in FIG. 7 c along lines A-A. The split-ring has a bottom surface190 oriented to engage the bottom surface 135 of groove 131. Thecross-section has an inclined upper surface 192 for engaging surface 133which is upwardly inclined on moving towards the center the split-ring.Preferably, the incline is at an angle of about 20° with respect tobottom surface 190. The surface forming internal diameter 145 is in twosections, the first is surface 194 which is generally parallel to axis164 of opening 102 and the second is surface 144 which is angledradially outwardly towards surface 192 also at preferably 20° withrespect to surface 194 (and the axis 164). Surfaces 192 and 144 arepreferably connected by a radius 198 rather than a sharp corner. Thepreferred split-ring has an overall height from the surface 190 to thetop of radius 198 of approximately 0.52 mm and the distance alongsurface 196 between surface 190 and 192 is about 0.29 mm.

The preferred cross-section allows spring-clip 104 to be assembledwithin groove 131 by the plate manufacturer and shipped to the user in apre-assembled condition. It is especially important that the clip 104have a sufficient number of cutout areas to render it sufficientlyflexible for insertion into the inserting recess or groove 131 prior toshipping to the end user. It is also necessary to use a relativelyflexible material for the ring, which material has a modulus less than100 GPa. Such a titanium material is found in U.S. Pat. Nos. 4,857,269and 4,952,236. If these titanium alloys are utilized for the split-ring,it has been found that advantageous to make the joining member or plateand anchoring or bone screw out of the same material, although such isnot absolutely necessary. In addition, polymeric materials can be usedfor the split-ring. In the preferred embodiment, the split ring 104 hasno means for enabling its removal from the groove after assembly. Thus,it is not possible for the surgeon to remove the ring from the plate.

Another advantageous feature of the split-ring is the preferably 20°incline of the top surface 192 which engages with complementary groovesurface 133. This is advantageous because forces generated from thebacking out of the screw 105 against the bottom surface 190 ofsplit-ring 104 tend to keep the inner diameter 145 from expanding. Inaddition, only a small annular inter-engagement between the bottomsurface 190 of ring 104 and the upwardly facing surface 156 is necessaryto prevent screw 105 from backing out of hole 2 in plate 1. In thepreferred embodiment, this annular overlap is at least 0.07 mm andpreferably between 0.07 mm on a radius and 0.11 mm.

In a third embodiment illustrated in FIG. 9, the device 201 differs froma first embodiment only in the shape of the blind holes 203 and of thecirclip 204 that can be housed in the recess 231. The shape of the holes203 has a semicircular base 237 continued by two straight surfaces 236which converge towards each other and are connected at their other endof the side of the associated orifices 2 by a semicircular vertex 235 ofsmaller radius than the base 237. This shape facilities the fitting ofthe circlip 204. The latter is very similar to the one in the firstembodiment, except for the lugs 241 which have holes 243 to take thejaws of a driving instrument. Installation with this embodiment isidentical to that of the first embodiment.

The circlip 104 may have a constant cross-section.

The bone screws may be monoaxial: they cannot be oriented with respectto the plate.

It can thus be seen that, in the embodiments of FIGS. 1 and 9, one andthe same circlip locks two anchoring screws.

In all these embodiments, each circlip collaborates by direct contactwith the screw to prevent it from coming out of the opening, without itbeing necessary to provide a part acting as an intermediate between thecirclip and the screw.

Referring to FIG. 10, there is shown a plan view of the preferredscrewdriver 300 for driving screw 105. The screwdriver 300 includes ahandle 302, a shaft 304 and a drive head 306. Referring to FIG. 10 a,there is shown an end view of drive head 306 showing a cruciform drivehaving a pair of mutually perpendicular blades 308. Blades 308 engagedrive 152 on screw 105. In the preferred embodiment, the depth of thecruciform slot forming drive 152 is about 2 mm and the depth of thedrive blades 308 is somewhat less and the width of the four slotsforming drive 152 are about 1 mm with the width of the blades 308 beingslightly less. This geometry ensures excellent engagement between theblades on the driver 300 and the drive 152.

Referring to FIGS. 11 through 11 b, there is shown a tool provided toremove the screw 105 after it has been fully inserted into bone andblocked from backing out by split-ring 104. Referring to FIG. 11 thereis shown an extraction tool 400 having a handle 402 and a tubular driveshaft 404, including a drive tip 406. Handle 402 is also tubular havinga cavity 408 open to an end 410 of handle 402. In the preferredembodiment, the cavity 408 is circular with a diameter of about 8 mm.Inner end 412 of cavity 408 is open to a cannulation 414 which extendsthe length of shaft 404 and through tip 406. In the preferredembodiment, this cannulation is circular with a diameter of about 2 mm.The function of cannulation 414 is described below.

Referring to FIG. 11 b, there is shown an enlarged view of drive tip 406of extraction tube 400 which, like driver head 306 previously described,includes a cruciform blade having cross members 416 similar to blades308. However, the outer diameter of tip 406 is equal to the outerdiameter of surface 156 on screw 105. Tip 406 includes an inwardlychamfered portion 418 which allows tip 406 to engage the inner diameter145 of the split-ring and expand it sufficiently to allow the screw tobe unthreaded or pulled back out through inner diameter 145 bycounter-rotation of screw 105 with extractor 400. Once the maximumdiameter of upwardly facing surface 156 passes through the split-ring,it springs inwardly along surface 154 of screw 105.

It has been found that in some instances, the bone deteriorates so thatit is impossible to generate a screw removal force by thecounter-rotation of screw 105 with extractor 400. In this instance,referring to FIG. 12, there is shown an extraction tool 500 designed tofit within the cavity 408 and cannulation 414 of extraction tool 400.Extraction tool 500 includes an upper portion 502, a shaft portion 504,a threaded tip 506 and an enlarged portion 508. The threaded tipincludes threads matching threads 160 in screw 105. In the preferredembodiment, the thread is 1.6 mm in diameter. Thus, when the bone screw105 cannot be removed merely by the counter-rotation of screw 105 withextraction tool 400, extraction shaft 504 is inserted through thecannulation 414 and out tip 406 thereof and into threaded engagementwith threads 160 of bone screw 105. All the surgeon must then do is pullon portion 508 of the extraction tool 500 which pulls screw 105 out ofthe bone.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. An implant comprising: a joining memberhaving a bone contact surface and an opposite upwardly facing surfaceand at least one opening with a groove formed around an innercircumference of the opening, said groove having an axially spaced upperand lower surface; a bone-anchor capable of being accommodated in theopening between said groove and said bone contacting surface; and anannular split-ring mounted in said groove, the annular split-ring sizedto expand upon insertion of the anchor element into the opening and tocome into direct contact with a head of the bone anchor to hold the boneanchor in the opening, said split-ring having an upper surface and alower surface for respectively engaging said upper and lower surface ofsaid groove, wherein the split-ring defines a distance between an innerand outer diameter of the annular split-ring, the outer diameter havinga plurality of cutout portions so that the distance between thesplit-ring inner and outer diameters has alternating wider and narrowerportions.
 2. The implant according to claim 1, wherein in the joiningmember comprises a plate.
 3. The implant according to claim 2, whereinthe openings comprise a spherical seat.
 4. The implant according toclaim 3, wherein the anchor comprises a complementary spherical partcapable of coming into contact with the spherical seat.
 5. The screwretaining system as set forth in claim 2, wherein said at least oneopening has a part spherical seat portion located between said grooveand a bottom portion of said plate and open to said surface to receive ashank of said screw.
 6. The screw retaining system as set forth in claim5, wherein said screw has a part spherical head portion extending fromsaid shank towards said upwardly facing surface for engaging saidspherical seat portion in said opening.
 7. The screw retaining system asset forth in claim 5, wherein said groove has a lower surface nearestthe bottom surface of the plate and an upper inclined surface extendingtoward an upper surface of said plate and inwardly toward a central axisof said opening.
 8. The screw retaining system as set forth in claim 7,wherein the groove lower surface is generally flat and extends generallyperpendicular to said central axis.
 9. The screw retaining system as setforth in claim 7, wherein said lower groove surface is located closer tosaid upper plate surface than an upwardly facing surface on said screwwhen said part spherical head is seated in said part spherical portion.10. The implant according to claim 1, wherein the anchor includes adrive portion.
 11. The implant according to claim 1 wherein the splitring has at least three wider portions separated by at least threenarrower portions.
 12. The implant according to claim 11 wherein thesplit-ring has five wider and narrower portions.
 13. The implantaccording to claim 1, wherein the split-ring is specific to eachopening.
 14. An implant comprising: a plate having a bone contactsurface and an opposite upwardly facing surface and at least one openingwith a groove formed around an inner circumference of the opening, saidgroove having an axially spaced upper and lower surface; a bone-anchorcapable of being accommodated in the opening between said groove andsaid bone contacting surface; a split-ring mounted in said groove, thesplit-ring sized to expand upon insertion of the anchor element into theopening and to come into direct contact with a head of the bone anchorto hold the bone anchor in the opening, said split-ring having an uppersurface and a lower surface for respectively engaging said upper andlower surface of said groove, wherein the split-ring has a variablewidth so as to optimize its flexibility; wherein said at least oneopening has a part spherical seat portion located between said grooveand a bottom surface of said plate and open to said bottom surface toreceive a shank of said screw; and wherein said groove has a lowersurface nearest the bottom surface of the plate and an upper inclinedsurface extending toward an upper surface of said plate and inwardlytoward a central axis of said opening.
 15. The screw retaining system asset forth in claim 14, wherein the groove lower surface is generallyflat and extends generally perpendicular to said central axis.
 16. Thescrew retaining system as set forth in claim 15, wherein said lowergroove surface is located closer to said upper plate surface than anupwardly facing surface on said screw when said part spherical head isseated in said part spherical portion.
 17. An implant comprising ajoining member having a bone contact surface and an opposite upwardlyfacing surface and at least one opening with a groove formed around aninner circumference of the opening, said groove having an axially spacedupper and lower surface; a bone-anchor capable of being accommodated inthe opening between said groove and said bone contacting surface; anannular split-ring mounted in said groove, the annular split-ring sizedto expand upon insertion of the anchor element into the opening and tocome into direct contact with a head of the bone-anchor to hold thebone-anchor in the opening, said annular split-ring having an uppersurface and a lower surface for respectively engaging said upper andlower surfaces of said groove, wherein the annular split-ring defines adistance between an inner and outer diameter of the annular split-ring,the outer diameter having a plurality of cutout portions so that thedistance between the annular split-ring inner and outer diameters hasalternating wider and narrower portions; and wherein the annularsplit-ring inner diameter is circular and the outer diameter hasportions spaced at two different distances from the inner diameter.